This invention relates to an amplifier having a stable gain for use at the tuner of a cable-connected TV and, more particularly, to a feedback amplifier for a cable-to-TV (hereafter, cable-TV) converter which can adjust easily and at the same time the amplifier gain, the distortion reduction, and the output impedance.
A cable TV system that receives broadcast television signals with an antenna having a high sensitivity and distributes the signals to homes using a wideband transmission line such as a coaxial cable has become popular. The cable TV system can not only retransmit the broadcast television signal, but also provide other information. In the latter case, at least, each receiving TV in the system requires a tuner for a cable-TV converter that can provide a stable gain.
Generally, the final stage of a cable-TV tuner, which sends an amplified video IF signal to the TV, has an amplifier stage, but no automatic gain control (AGC), and gain deviations from one converter in the system to another cannot be controlled. Thus, if the gain of one converter is larger, its distortion increases, too, due to the increment of gain. In such case, stable gain cannot be obtained by controlling the gain of only the final stage. Therefore, the tuner gain must be controlled additionally for distortion reduction, while maintaining the proper output impedance and keeping down the size and weight of the cable-TV converter.
As shown FIG. 1, which shows the final amplifier stage of the tuner of a conventional cable-TV converter, the final-stage IF amplifier 1 is connected between an input port 3, which receives the video IF signal, and a variable pad 2. The variable pad 2 controls the gain of the tuner and provides the amplified video IF signal to an output port 4. In such a final amplifier stage, the IF amplifier must have a gain large enough to avoid gain deviation, while the desired gain range is satisfied using the variable pad 2. For this, a cascaded bipolar transistor is used for the amplifier 1 to provide a large signal gain and the variable pad 2 is connected for reducing the distortion to the output port 4.
In this previous technology, the variable pad 2 consists of an independently variable resistor. Another variable pad (not shown) can be used at the input to the amplifier, too, to reduce the distortion when the amplifier has the large gain desired as explained above. As a result, too many circuit components are needed, at least for a cable-TV converter, which requires small size and weight. Further, the design of the converter becomes too complex and its fabrication cost too high on the account of the many circuit components that are used. Furthermore, the output impedance of the converter is not well maintained while controlling the gain of an amplifier.